Please use this guide instead, this one was written in a hurry and I forgot I had the older one.
Basically, you'll want to start with the MCP 1.12 release (temporarily at http://www.modcoderpack.com/files/mcp940.zip ), and then modify it to work with 1.12.2. This is actually quite simple.
First, you need to edit conf/version.cfg and set both versions to 1.12.2. That on its own is not enough, but it's a critical step. Then you need to grab new SRG mappings. Normally you'd get that through the !srg command, but MCPBot doesn't have it for some versions.
Instead, you can get it through forge's maven. (normally it's also on the export site but not for 1.12.2). The link is http://files.minecraftforge.net/maven/de/oceanlabs/mcp/mcp/1.12.2/mcp-1.12.2-srg.zip .
To set that up, first delete the conf/patches folder (otherwise patches will be merged, breaking things), and then unzip the contents of that ZIP into conf, overwriting files as prompted.
Then, grab a fresh set of MCP mappings. You can get this just via the export site: http://export.mcpbot.bspk.rs/ . Grab the latest fields, methods, and params CSV files, and place those in conf as well. That's all! You can now run the normal MCP scripts, and it'll work for that version.
Note that as always, the MCP license applies, even for development builds. Please read the
LICENSE.txt included with MCP for more information about what you are and are not permitted to do.